1. Field of Invention
The invention relates to ballistic ceramic armor. More particularly, this invention pertains to a ballistic ceramic armor manufactured from a composite ceramic material product, and a method for manufacturing the same.
2. Description of the Related Art
In order to provide protection of personnel and equipment from ballistic projectiles, explosive ordnance, and forces and objects from detonation of improvised explosive devices (collectively hereinafter “projectiles”), it is necessary to provide a means of disbursing the kinetic energy of such projectiles to prevent them from reaching their target. Although this may be accomplished by interposing a large mass of any of a number of different materials between the target and the incoming projectile, experience has shown that a much more efficient means of energy disbursement is provided by suitably engineered ballistic armor structures wherein at least one layer of material acts to disrupt an incoming projectile. Such a structure strives to maximize the amount of material which may be acted upon to absorb and disburse the energy of the projectile, while at the same time breaking or deforming the projectile and distributing these resulting fragments into a wider area. Such a structure further strives to minimize the total amount of materials required for the protection of a specific area.
Ballistic armor structures generally include one or more layers of material engineered to spread the force of the impact by deforming, deflecting, or fragmenting the ballistic projectile while the ballistic armor itself undergoes deformation or localized fragmentation. The deformation and localized fragmentation processes of the ballistic armor structure absorb a large portion of energy from the projectile while simultaneously spreading the impacted area to involve more material in successive layers. Both hardness and toughness of the ballistic armor structure are required for these functions.
In recent decades, certain hard ceramic materials have been developed for certain ballistic armor applications. These ballistic ceramic materials, such as alumina, boron carbide, silicon carbide, and titanium diboride ceramics provide the advantage of being lighter in mass than steel and provide ballistic stopping power comparable to steel. Ballistic ceramics are extraordinarily hard, strong in compression, and relatively light weight, making them efficient at eroding and shattering armor-piercing threats. Thus, in applications in which having armor with the lowest possible mass is important, such as human body armor and aircraft armor, ballistic ceramic materials are useful.
A common method of manufacture of structures made of ballistic ceramic materials is to sinter components of ballistic ceramic materials to form the structure using hot pressing. In a hot pressing process, particles of ceramic material are subjected to elevated temperature and then subjected to increased isostatic gas pressure in an autoclave. An inert gas is used to discourage chemical reaction of the ceramic material. The increased temperature causes the ceramic material to undergo a process called sintering, whereby the particles adhere to each other. Thereafter, the increased pressure and temperature encourages grain boundary diffusion to allow for increased densification of the structure.
In using a hot pressing process to manufacture ballistic ceramic materials, pressures exceeding 2,000 psi and temperatures in the range of 1,500 to 2,240 degrees Centigrade, depending on the particular constituent materials used, are necessary to achieve sufficient adhesion of the ballistic ceramic particles. The need to achieve and maintain such high temperatures and pressures makes manufacture of hot pressed ballistic ceramics a costly endeavor, thereby resulting in increased cost to the consumer of the ballistic ceramic material.
Reaction bonding has been used as an alternative in manufacturing ballistic ceramic structures. In reaction bonding, a composite is formed of ceramic particles bonded in a matrix of in situ formed ceramic material. In this process, ceramic particles are mixed with carbon and a sintering aid, such as silicon. The mixture is then heated to a point at which a portion of the carbon and the sintering aid react to form a composite ceramic consisting of ceramic particles distributed throughout a matrix of ceramic material. This reaction results in a semi-continuous phase of sintering aid distributed throughout the composite, with discontinuous ceramic material phases bonding discontinuous phases of ceramic particles.
The reaction bonding process poses several attractive advantages to manufacture of ballistic ceramic structures over hot pressing. Less pressure and temperature are necessary to carry on the reaction bonding process as opposed to hot pressing, thereby making reaction bonding more economical. Also, reaction bonding is accomplished using the relatively inexpensive raw materials of the ballistic ceramic materials, such as silicon and carbon together with the ballistic ceramic particles.
However, despite the advantages of reaction bonding, the performance and quality of reaction-sintered ballistic ceramic composite material has traditionally been deemed inferior to hot pressed ballistic ceramic material. Ceramics inherently contain flaws such as micro cracks, porosity, voids, impurities, and residual stresses from processing that can serve as sites for initiation of failure by mechanisms such as intergranular flow, sliding and micro cracking within the initial or reflected stress wave states. U.S. Pat. Nos. 7,104,177; 6,995,103; and 6,862,970 each disclose the use of silicon as an agent to react with carbon and form silicon carbide as a phase that bonds a filler ceramic, either boron carbide or silicon carbide, together with approximately 10-20 percent of unreacted silicon remaining in the composite. Certain publications have theorized that this amount of excess silicon is deleterious to the ballistic performance of the finished ceramic material. As is set out in V. Domnich and Y. Gogotsi, Phase Transformation in Silicon Under Contact Loading, Rev. Adv. Mater. Sci. 3, 1-36 (2002), the amount of excess silicon ultimately leads to void formation and failure sites upon impact by a ballistic projectile.
Moreover, in traditional reaction bonding, organic materials, such as graphite, are added to the suspended ballistic ceramic particles without assuring that the organic materials would cover the surface of all suspended ballistic ceramic particle grains. As a result, the reaction bonded silicon carbide occurs in a discontinuous phase throughout the composite, with uneven distribution of silicon carbide and relatively low surface area contact between the silicon carbide and filler particles. This lack of uniformity of silicon carbide distribution leads to imperfections within the ceramic composite, which in turn leads to decreased strength and toughness of the ceramic composite. Thus, quality control of the resulting ceramic composite is difficult to maintain using traditional reaction bonding techniques. Furthermore, the use of silicon as a continuous phase is hampered by the tendency for molten silicon to vaporize at temperatures above 1,414 degrees Centigrade. As silicon vapor escapes the colloidal mixture during formation of the ceramic product, it leaves behind degassing channels, which weaken the structural integrity of the ceramic product.
In order to apply reaction bonded ceramic composites to a ballistic armor application, more uniform bonding between the suspended ballistic ceramic particles and the in situ formed ceramic matrix is important. Ceramic bodies tend to exhibit stronger and more reliable properties when they are uniformly fine grained, fully dense, and non-porous. Microstructure is known to influence dynamic properties thought to relate to dynamic yield strength. For example, N. K. Bourne, et al. “The Effect of Microstructural Variations upon the Dynamic Compressive and Tensile Strengths of Aluminas” Proceedings: Mathematical and Physical Sciences, Vol. 446, No. 1927 (Aug. 8, 1994), pp. 309-318, discloses that smaller grain size corresponds to a higher Hugoniot Elastic Limit (HEL) in ceramic materials. In sintered ceramics, a higher glassy grain boundary phase is associated with lower strength, thus indicating that a ceramic with smaller grain size and reduced amount of second “bonding” phase exhibits improved dynamic properties and resistance to failure in dynamic impact conditions. Thus, suspending small particles of a relatively hard ballistic ceramic material, such as boron carbide, within a uniform and substantially continuous phase of softer yet tougher ceramic material, such as silicon carbide, would allow for a composite ballistic ceramic material capable of exhibiting increased overall strength and toughness.
As has been mentioned above, boron carbide and silicon carbide are sometimes used as filler materials for ballistic ceramic composites. However, other possible fillers are known. In particular, under certain conditions boron carbide (chemical formula B4C) reacts with silicon to form a crystal lattice in which silicon atoms and boron atoms are substituted for some of the carbon atoms within the lattice. The substitution of larger silicon atoms for carbon atoms at certain positions in the lattice framework results in an expanded lattice with the chemical formula B12(B,C,Si)3. The expansion of the crystal lattice also allows for the insertion of interstitial atoms into some of the gaps in the lattice framework. Overall, the expansion of the lattice and the interstitial insertion of atoms create a filler material having physical properties more desirable for ballistic armor applications than ordinary boron carbide. See R. Telle, “Structure and Properties of Si-Doped Boron Carbide,” The Physics and Chemistry of Carbides; Nitrides and Borides (Ed. by R. Freer, Kluwer Academic Publishers, Netherlands, 1990), 249-67. Hereafter, the expanded lattice formed from the reaction of silicon with normal boron carbide will be referred to as “expanded lattice boron carbide.”
In manufacturing a reaction bonded ballistic ceramic composite having a fine grained filler distribution; a problem arises in appropriating a filler material of sufficiently small grain size. Specifically, finely divided ceramic material suitable for use as filler is significantly more expensive than large grained ceramic material of equal purity. Moreover, a second problem arises in disbursing organic materials evenly throughout the fine grained filler material. In traditional processes of dividing a large grained ceramic material into a more fine grained material, the opportunity for contaminants to adhere to the surface of the filler granules is great. Similarly, in traditional processes of dividing carbon and other organic materials into material of sufficient grain size to allow for disbursement within the fine grained filler material, the opportunity for contaminants to oxidize portions of the organic material is great. Such contamination of the fine grained filler material and the organic material leads to reduced bonding between the filler material and the organic material, thereby reducing the amount of surface to surface contact between the continuous-phase in-situ-formed ceramic material and the filler material. Such reduced surface-to-surface contact ultimately results in greater instances of weak points in the product ceramic composite.